As it is well known, the power delivered by an internal-combustion engine depends on the amount of air fed into the combustion chamber of this engine, and this amount of air is itself proportional to the density of this air.
If high engine power is required, this amount of air is increased by compression of the air by means of a supercharging device (turbosupercharger or driven compressor such as a screw compressor) before it is fed into the combustion chamber.
In order to increase this amount of compressed air allowed to pass into the cylinder, a stage of scavenging the residual burnt gas present in the combustion chamber is provided. This scavenging stage allows, before the end of the engine exhaust cycle, to discharge the burnt gas present in the combustion chamber and to replace it by compressed (or supercharged) air.
As described in patent U.S. Pat. No. 4,217,866, this scavenging stage consists in carrying out, at the end of the exhaust cycle and at the start of the intake cycle of a cylinder, overlapping of the exhaust and intake valves of this cylinder. This overlap is obtained by opening simultaneously these exhaust and intake valves for some degrees to some ten degrees of crankshaft rotation angle.
In the indirect fuel injection supercharged engine example described in this document, a burnt gas exhaust means with an exhaust valve controlling an exhaust pipe and two intake means are provided. One of the intake means is a non-carbureted supercharged air intake means with a pipe and a valve, and the other intake means is a carbureted air intake means consisting of a pipe carrying a fuel injection nozzle and a valve.
Burnt gas scavenging is carried out, on the one hand, by overlap of the exhaust valve and of the non-carbureted supercharged air intake valve, the carbureted air intake valve remaining closed and, on the other hand, by the fact that the pressure of the air at the open intake valve is higher than the pressure of the exhaust gas still present in the combustion chamber The non-carbureted supercharged air that is allowed into the combustion chamber scavenges this gas and discharges it through the exhaust valve. Non-carbureted supercharged air therefore occupies the volume freed by this exhaust gas, which allows a significant increase in the amount of air introduced during the engine intake cycle. In the vicinity of the end of the scavenging stage, the exhaust valve closes, the carbureted supercharged air intake valve opens, the fuel injection nozzle is activated and carbureted supercharged air is fed into the combustion chamber through the other intake means as a complement to the non-carbureted supercharged air that is allowed therein.
Although this type of engine runs satisfactorily, it however involves drawbacks that are by no means insignificant.
In fact, during the burnt gas scavenging stage, it is necessary to use a single control means controlling the two intake valves, such as those known as VVT (Variable Valve Timing), which allow to vary the lift laws of these valves, in particular at their time of opening/closing.
A camshaft with actuating cams having each a very precise profile and with an angular shift in relation to one another, which allow to work with and without a scavenging stage, notably under engine idle speed conditions, is provided.
Such a camshaft type is delicate to achieve and it requires many attempts to obtain the suitable profiles and angular shifts. Furthermore, it has a relatively great apparent spread.
Besides, outside the scavenging stage, the offset cams penalize running of the engine, notably its filling at high speeds. Furthermore, to obtain this cam shift, it is necessary to reach a compromise between the engine running mode with scavenging stage and the running mode without scavenging stage.
The present invention aims to overcome the aforementioned drawbacks by means of an engine with devices allowing the valves to be actuated without requiring complex and expensive control means.